1. Field of the Invention
The present invention relates to absorbed type lightning rod which absorbs lightning discharge current and which discharges the current to the ground to prevent the damage due to lightning, and an absorbed type lightning discharging apparatus utilizing the lightning rod.
2. Description of Related Art
When the thunder clouds approach, a conductive sharp material at the ground starts discharge to the thunder clouds (corona discharge or upward streamer), and the thunder clouds also start discharge to the conductive sharp material (downward streamer). Further, when insulation of air is lost under some weather condition, both discharges are coupled (called coupled streamer). This phenomenon is called cloud-to-ground discharge or lightning stroke. As the tip of the sharp material is more acute, the discharge to the thunder clouds occurs more.
Some conventional lightning rods are shown in FIGS. 1A, 1B, 2, 3A, 3B. As shown in FIGS. 1A and 2, a lightning rod B receives the lightning discharge current E (hereinafter called lightning stroke current) due to the lightning stroke X at a point, and discharges the current E to the ground. The lightning rod B also splashes the lightning stroke current, which means the lightning rod cannot absorb, (hereinafter called splashed lightning current E1) outsides the certain area (safety area). From the research results, it is well known that the corona discharge occurs more and lightning absorptivity becomes higher as the point of the lightning rod B is more acute. Accordingly, the point of the lightning rod B is preferably acute, and generally, "Franklin Rod" that is a circular cone-shaped pointed rod is used.
The lightning rod B has higher lightning induction and higher lightning absorptivity as the point thereof becomes more acute; however, the splashing area of the splashed lightning current E1 becomes narrow. Therefore, the point angle is made approximately 90 degrees to secure.
The modified lightning rod is disclosed in Japanese Patent Laid-open No. Hei 5-121192. This rod is that as shown in FIGS. 3A, 3B, a ring B1 is provided at the middle of the lightning rod B and that the angle between the point of the lightning rod and the circumference of the ring B1 (hereinafter called splash angle) is in the range of 90-180 degrees. For example, in FIG. 3A, the splash angle is 90 degrees, and in FIG. 3B, it is 126 degrees. In this lightning rod B, the point angle of the lightning rod B1 is made 60 degrees to maintain the lightning induction and the safety area widen owing to the ring B1; however, since the splashed lightning current E1 still exists, the secondary disaster due to the splashed lightning current E1 struck the lightning rod B1 cannot be avoided.
The modified lightning rod is also disclosed in Japanese Laid-open No. Hei 1-10600 and French application No. 9212860. The object of these inventions is to enhance the lightning induction by modifying the shape of the point of the lightning rod but the prevention of the secondary disaster is not the object.
The lightning stroke current conducted to the ground with the lightning rod B momentary becomes large, and very high potential is generated at an earth electrode C in the ground (see FIG. 2). The conventional earth electrode C is a thin metal plating rod having a conductive plate as a primary electrode and it is driven to the ground until the desired earthing resistance (Japanese standard is less than 10 ohms) is obtained. Accordingly, the earthing resistance in conformity with the standard is obtained but the instantaneous lightning stroke current cannot instantaneously and smoothly be discharged to the ground. In result, the lightning stroke current is stopped at the earth electrode C and flows back to the lightning rod B (see FIG. 1B). This reverse current E2 generates the flush over and the excess electromagnetic induction. Moreover, it generates the Joule's heat, which induces the tertiary disaster such as the damage of the lightning rod B.
Further, since the earth electrode C such as a conductive plate and a metal rod is only driven to the ground, the some space is formed between the earth electrode and the ground, which decreases the discharge effect.
The lightning stroke current absorbed by the lightning rod flows through the surface of the conductive wire. Conventionally, since the lightning rod and the earth electrode are coupled only with a conductive wire, the lightning stroke current is leaked from the surface of the conductive wire. If the object to be protected is a building, the leakage current is not a problem. However, if the object to be protected is an antenna or electric line, the leakage current causes the bad influences to the object to be protected.